Understanding Go's net/http package

Accepting HTTP requests is the primary goal of a web server. In Go, there is a system-level package that helps developers create HTTP servers and clients. The name of the package is net/http. We can understand the functionality of the net/http package by creating a small example. The example accepts an incoming request and returns the timestamp of the server. Let us see the steps for creating such a server:

1. Create the program file as follows:

touch -p $GOPATH/src/github.com/git-user/chapter2/healthCheck/main.go

Now, we have a file where we can develop a server with a Health Check API that returns a date/time string.

2. Import the net/http package and create a function handler called HealthCheck. The http.HandleFunc is a method that takes a route and a function handler as its arguments. This function handler has to return an http.ResponseWriter object:

package main

import (
 "io"
  "log"
  "net/http"
  "time"
)

// HealthCheck API returns date time to client
func HealthCheck(w http.ResponseWriter, req *http.Request) {
  currentTime := time.Now()
  io.WriteString(w, currentTime.String())
}

func main() {
  http.HandleFunc("/health", HealthCheck)
  log.Fatal(http.ListenAndServe(":8000", nil))
}

The preceding code creates a HealthCheck function and attaches it to an HTTP route. HandleFunc is used to attach a route pattern to a handler function. ListenAndServe starts a new HTTP server. It returns an error if the server launch is unsuccessful. It takes address:port as the first argument and the second argument is nil, which says use the default multiplexer. We will see multiplexers in detail in the upcoming sections.

Use the log function to debug potential errors. The ListenAndServe function returns an error if there is one.

3. Now, we can start the web server using this command:

go run $GOPATH/src/github.com/git-user/chapter2/healthCheck/main.go

Run the healthCheck.go file from a shell.

4. Now, fire up a shell or browser to see the server in action. Here, we use the curl request:

curl -X GET http://localhost:8000/health

The response is as follows:

2019-04-10 17:54:05.450783 +0200 CEST m=+6.612810181

Go has a different concept for handling request and response. We used the io library to write to the response. For web development, we can use a template to automatically fill in the details. Go's internal URL handlers use a ServeMux multiplexer. In the next section, we will discuss more on ServeMux, a built-in URL router in Go.